Root Elongation of Soybean Genotypes in Response to Acidity Constraints in a Subsurface Solution Compartment

Abstract

Aluminum‐tolerant germplasm is needed to overcome subsurface acidity constraints to root growth and plant access to water and nutrients. Root elongation of four soybean [Glycine max (L.) Merr.] genotypes exposed to varying concentrations of Al, H, and Ca were compared in two experiments using a vertically split root system. Roots extending from a limed surface soil compartment grew for 12 d into a subsurface compartment with nutrient solution treatments. In Exp. 1 root growth for cv. Ransom and Plant Introduction 416937 (PI) were compared in solutions with factorial combinations of pH (4.2, 5.2) and Al (0, 7.5, 15 μM) with Ca maintained at 10 mM In Exp. 2 soybean line N93‐S‐179 (N93), PI, and cultivars Ransom and Young were compared in solutions with factorial combinations of Ca (2 and 10 mM) and Al (7.5 and 15 μM) maintained at pH 4.6. Ransom and PI had similar responses in tap and lateral root elongation to solution pH and Al treatments in Exp. 1, but mean tap root length of Ransom in the subsurface compartment exceeded that of PI by 22%. Aluminum inhibited the length of lateral roots more than tap roots in both experiments. Molar activity ratios between Ca and Al3+ {Ca/Al3+} accounted for most of the differences in root elongation response among solution treatments in Exp. 2. A 50% reduction in relative length of tap roots for all genotypes occurred with a {Ca/Al3+} value of 891. Values of {Ca/Al3+} for 50% reductions in relative length of lateral roots differed among genotypes and were 1.6 to 3.5 times greater than for tap roots. On the basis of the {Ca/Al3+} indices for lateral root length, line N93 and Ransom exhibited greater tolerance to subsurface solution Al than PI and Young.